The tradeoffs and requirements of radiation sources for electromagnetic and optical radiation sources, and in particular the use of enclosed electrically-excited filaments, have been the subject of development for over 100 years. As this development addressed more narrow and specific radiation requirements of controlled wavelength emission for accuracy and precision, power efficiency requirements for economy, and loss reduction and temperature control, the problems involved in design and manufacture of suitable radiation sources have become correspondingly more complex.
A particular application environment that has received a great deal of inquiry is the area of infrared radiation, which is efficiently useful and necessary in a variety of measurement and detection instrumentation. Many such applications are limited in power, space and cooling ability and require efficient illumination within a limited spectral band. Some considerations of this environment and difficulties of emitter design are discussed in U.S. Pat. No. 3,875,413 to Bridgham for Infrared Radiation Source, which particularly recognizes the difficulty of achieving stability and control of temperature and emission wavelength in a thin, flat, electrically heated radiator.
Temperature stability has been a particular development objective of traditional IR sources for calibration and measurement applications, which rely on steady state heating of an object with relatively large thermal mass. This in turn requires a long turn-on and settling time for stable operation and produces a large amount of waste heat.
As will be seen in the following descriptions, the invention may be compared favorably as an improvement over many previous radiation sources and could usefully replace such traditional reference emission sources as wire filament bulbs, LEDs, lead salt lasers, and rare earth oxide line emitters in measurement applications. Although these narrow band emitters produce isolated line radiation, they can only be tuned with difficulty and over narrow ranges. Incandescent sources typically produce a radiation spectrum described by the Planck curve with very little of the total radiation in the desired band for a particular measurement. Specifically, again in the infrared field, sources of the prior art include developments such as pulsed radiation sources using a thin plate form of radiation filament.
The prior art generally teaches the necessity of a thin plate element for radiation cooling, the '413 patent referenced above, for instance, specifying 1-2 .mu.m. U.S. Pat. No. 5,220,173 to Kanstad for Pulsating Infrared Radiation Source proposes a formula for required thinness. The '173 patent proposes that thin flat plate elements will efficiently radiate in the IR range as the low mass of the thin material will radiate greater heat than stored thermal energy delivered by a pulsed driving circuit, and predicts the thinness of material necessary to produce this effect at the 1-2 micron range. As the focus of the prior art is on radiation source thinness for cooling effect, problems of emissivity, wavelength control and resistance control have been unaddressed.